The present invention relates to an electrophotographic photoreceptor and an image forming method using the same, and more particularly for the electrophotographic photoreceptor well adaptable for an electrophotographic printer in which the line scan by a laser beam is used for image formation, and an image forming method using such an electrophotographic photoreceptor.
In an electrophotographic printer of the type in which the line scan is carried out by using a laser beam, a gas laser of a relatively short wavelength, such as a helium-cadmium laser, argon laser, and helium-neon laser, is used for generating the laser beam. An electrophotographic photoreceptor, which is used in combination with the gas laser, uses a CdS-binder photosensitive layer and a charge transfer complex (IBM Journal of the Research and Development, Jan. 1971, pp. 75 to 89), which corporate to form a thick photosensitive layer. With such a structure, there occurs no multiple reflection of the laser beam within the photosensitive layer. An image as formed is free from a pattern of interference fringes.
Recently, a semiconductor laser has gradually superseded the gas laser, because of recent design trend of reducing cost and size of the electrophotographic printer. The semiconductor laser generally requires the electrophotographic photoreceptor whose sensitivity is high in a region of long wavelengths. The electrophotographic photoreceptor has also been developed so as to have such a sensitivity characteristic.
For the photosensitive members with good sensitivity for radiation of long wavelengths for example, not more than 600 nm, which have been known, there may be enumerated the electrophotographic photoreceptor using the photosensitive layer containing phthalocyanine pigment, such as copper phthalocyanine, aluminum chloride phthalocyanine, particularly the electrophotographic photoreceptor of the multi-layer type including a multi-layer photosensitive layer consisting of a charge generating sub-layer and a charge transfer sub-layer, and the electrophotographic photoreceptor using a selen-tellurium film.
Let us consider a case that the electrophotographic photoreceptor of such a photosensitive characteristic is coupled with the electrophotographic printer of the laser beam scan type, and a laser beam exposure is applied thereby to form a toner image. In this case, an interference fringe pattern appears in the toner image. The resultant reproduced image is poor.
One of the causes to produce the interference fringes follows. The laser beam of long wavelength is incompletely absorbed within the photosensitive layer, part of the laser beam is transmitted through the photosensitive layer, and is reflected on the substrate surface. Accordingly, a multiple reflection of the laser beam is caused within the photosensitive layer. The multiple reflected laser beam interferes with light reflected on the surface of the photosensitive layer.
There have been proposals to solve the multiple reflection within the photosensitive layer. A first proposal is to rough the surface of the conductive substrate in an electrophotographic photoreceptor layer by anodic oxidation treatment or buffing, as disclosed in Japanese Patent Unexamined Publication Nos. Sho. 58-162975, Sho. 60-79360, Sho. 60-112049, Sho. 61-42663, and Sho. 62-186270. A second proposal is to interlayer a light absorbing layer or a reflection preventive layer between the photosensitive layer and the substrate, such as disclosed in Japanese Patent Unexamined Publication Nos. Sho. 58-17105, Sho. 59-158, Sho. 59-204048, Sho. 60-86550, and Sho. 62-150259. A third proposal is disclosed in Japanese Patent Unexamined Publication No. Sho. 58-82249 in which most of light from a light source is absorbed by the charge generating layer. A fourth proposal is to quench the multiple reflection by using the substrate as subjected to colored anodized aluminum.
Actually, however, the proposals as mentioned above fail to completely remove the interference fringes produced at the time of forming an image. Particularly, in the first proposal to make the substrate surface irregular, it is difficult to uniformly rough the substrate surface. Accordingly, a relatively thin irregularity is present in a local area, which occupies a specific ratio of the substrate surface. In this case, the thin irregularity area serves as a carrier injection part for the photosensitive layer. This causes white spots at the time of image formation (in the reversal development method, black spots appear in the image). In this point, the first proposal is disadvantageous. For only the interference fringes problem, there are many solutions. For both the problems of the interference fringes and of the white spots or black spots, it is very difficult to find good solutions. In the proposal to rough the surface of the conductive substrate, difficulty exists in manufacturing a lot of the substrates whose surfaces have uniform irregularity. Thus, the first proposal involves problems to be solved.
In the case of the second proposal using the underlayer with a diffuse reflection surface between the conductive substrate and the photosensitive layer, it is difficult to intentionally control the irregularities on the irregular surface of the underlayer, and further to reproduce the same irregular surface. To rough the surface of the underlayer to such an extent as to effectively prevent the interference fringes, a large thickness is required for the underlayer. The thick underlayer adversely affects the electrophotographic characteristics, such as a sensitivity of the photosensitive layer, adhesiveness, and the like. Use of the underlayer that makes the structure of the electrophotographic photoreceptor complicated leads to increase of the cost to manufacture.
The proposal to absorb most of light from a light source by using the charge generating layer may be realized by (1) increasing the thickness of the charge generating layer, (2) making the peak of the spectral absorption of the charge generating layer approximate to the wavelength of the light source light, or (3) using pigment to absorb the light source light. In the case of (1) above, if the charge generating layer is made thick to completely remove the interference fringes, it is unsuitable for the electrophotographic operation. Specifically, the thermally excited carriers are increased, and adversely affects the dark attenuation and the acceptance potential. In the case of (2) above, it is difficult to find materials having such a nature. If found, few materials have satisfactory performances, and it is difficult to effectively use the materials. If the absorption peak of the material overlaps with the wavelength of the light source light, the material has a limit in its absorption. In the case of (3), there is the possibility that use of the pigment has an adverse effect on the electrophotographic characteristics. Further, few pigments having no adverse effect exist.
Let us consider the proposal using a light absorbing layer between the conductive substrate and the photosensitive layer. In this proposal, as of the selenium photosensitive member disclosed in Japanese Patent Unexamined Publication No. Sho. 62-150259, the light absorbing layer for absorbing light of a specific wavelength must be additionally layered on the substrate which is polished and subjected to etching process. The additional use of the absorbing layer makes the layer structure complicated and increases cost to manufacture.
The proposal using the conductive substrate as subjected to colored anodized aluminum is allowed to be applied for only the substrate that is made of metal. To prevent the generation of the interference fringes pattern, it is necessary to use a thick anodic aluminum. Use of the thick anode aluminum deteriorates conductivity and hence, the electrophotographic characteristic of the electrophotographic photoreceptor.